1. Field of the Invention
The invention relates to digital and analog correlators particularly with respect to enhancing the accuracy thereof.
2. Description of the Prior Art
Digital as well as analog correlators are known in the art for correlating two signals and providing a correlation output therefor. Generally, in a digital correlator, digitally converted samples of the signals are applied to a digital multiplier and the product signals therefrom are applied to a digital accumulator for generating the correlation output. In an analog correlator, the analog signals are multiplied and the products are applied to an integrator for generating the correlation output.
In such prior art systems, the input data is multiplied by a constant value prior to performing the multiplication in order to preserve the precision therein. This procedure increases the numerical value of the input data thereby enhancing the precision. In the digital correlator, multiplication by the constant value shifts the information carrying bits to higher significant digit positions thus preserving the information from loss due to lower significant bit truncation in the multipliers. Typically a multiplication factor of 32 (a left shift of 5 bits) may be utilized. In the analog correlator, the multiplicaton factor is applied to raise the information content of the input signals above the inherent noise level of the components of the correlator.
Frequently, in the digital correlator, the multiplication by the constant factor creates sufficiently large amplitude data to cause an overflow in the multiplication or accumulation operations. If the overflow is not detected the correlation output is erroneous and cannot be utilized. Generally, it is undesirable to endeavor to detect the overflows in a digital correlator. Since, typically, the overflows occur in the accumulator carry stage, each time an accumulation is performed the carry bit must be checked. This results in a substantial loss in processing time which in many applications cannot be tolerated. Additionally, the equipment may not include a stage for receiving the overflow and hence detection thereof would not be possible. In the analog accumulator, multiplication by the constant factor may create sufficiently large amplitude data to cause nonlinear operation, such as amplifier saturation, in the components of the correlator. Such anomolous operation tends to render the correlation output useless.